Chemistry - EDEXCEL IGCSE - Organic Chemistry

ORGANIC CHEMISTRY:

Homologous Series - a family of compounds with similar chemical properties, a trench in physical properties, and the same general formula e.g. alkanes

Hydrocarbon - compounds that only contain hydrogen and carbon (Note. the word only is very important)

Saturated - contains the maximum number of hydrogen atoms possible within the given number of carbons

Unsaturated - does not contain the maximum number of hydrogen atoms possible because of double carbon-carbon bonds.

General formula - a molecular formula for a whole homologous series

Isomer - when two or more different compounds have the same molecular formula but different structural formula they are called isomers.

Alkanes:

General formula - C_nH_2n+2 

Reactions involving alkanes:

Combustion - e.g CH₄ + 2O₂ —> CO₂ + 2H₂O

Incomplete combustion - e.g. 2CH₄ + 3O₂ —> 2CO + 4H₂O

Methane reacts with bromine gas in the presence of UV light to form bromomethane and hydrogen bromide gas. This is a substitution reaction

CH₄ + Br₂ —> CH₃Br + HBr

Alkenes:

General formula - C_nH_2n 

Alkenes have an addition reaction with bromine, the equations are:

e.g. CH₂=CH₂ + Br₂ —> CH₂BrCH₂Br

This reaction is a classic test for alkenes. By passing an unknown organic compound through bromine water, if the orange bromine water is decolorised then there is a presence of a carbon-carbon double bond, a feature of alkenes. If, however, a alkane is present then the bromine water will remain orange and alkanes do not contain carbon-carbon double bonds. 

Ethanol:

Manufacture of ethanol:

1. Hydration of ethene - steam and ethene is constantly passed over the catalyst of concentrated phosphoric acid at 300°C and at 60-70 atmospheres. This is a continuous flow process, which makes it very efficient. It also produces ethanol of far higher purity.

   CH₂=CH₂ + H₂O —> CH₃CH₂OH(g)

2. Fermentation - yeast is added to sugar or starch solution, left in warm conditions (30 - 40°C) and in absence of oxygen (anaerobic conditions). Enzymes convert the sugar into ethanol and CO₂.

    C₆H₁₂O₆ —> 2C₂H₅OH + 2CO₂

Dehydration of ethanol:

Ethanol vapour is passed across aluminium oxide (catalyst) with heat

CH₃CH₂OH —> CH₂=CH₂ + H₂O

This also works for other types of alcohol 

CH₃CH₂CH₂OH —> CH₃CH=CH₂ + H₂O